The invention broadly relates to a process for preparing pure silicon dioxide by a two step ammonium hydroxide conversion of hexafluorosilicic acid solution and separating off the precipitated silicon dioxide.
Fluorine containing gases are typically obtained as undesirable by-products during the manufacture of fertilizer, phosphoric acid and the like. For instance, silicon tetrafluoride is often liberated in such processes but may not be discharged to the surrounding environs because of environmental concerns. It therefore must be recovered to prevent environmental problems. However, the recovered and collected silicon tetrafluoride by-product has heretofore generally been regarded as having little commercial value. Consequently efforts have been made to find a economically attractive use for such undesired but collected by-products.
One such attempt is described in U.S. Pat. No. 3,271,107 wherein a 15 to 25 weight percent hexafluorosilicic acid solution is reacted with ammonium hydroxide to produce a product slurry or suspension including numerous extremely small silica nuclei in a first step in a reaction zone of high agitation until a pH of 6-8 has been reached. The thus formed slurry is then treated in a second step for 20 to 120 minutes with less vigorous stirring in the presence of ammonium hydroxide at a pH of 8.3-9.0 and the precipitated silicon dioxide is subsequently separated off from the resulting slurry.
Disadvantageously, this already known process produces a badly contaminated silicon dioxide flocculate product which has limited end-use potential. For instance, the metal content in particular is undesirably high. In practice the thus obtained relatively high metal content flocculate can therefore be used for comparatively few technical purposes such as a paint pigment additive or a rubber product filler.
The present invention now provides a process which makes it possible to obtain a pure silicon dioxide from even highly contaminated hexafluorosilicic acid solutions.
The present invention also provides an improved process for preparing pure silicon dioxide which is suitable for many useful technical purposes.